1. Field of the Invention
The present invention relates to fluidic control systems and particularly to systems wherein a monitored pressure or pressure ratio is employed to control a servo valve. More specifically, this invention is directed to servo valves especially suited to use in such control systems and characterized by positive feedback, override capability and hysteresis adjustment. Accordingly, the general objects of the present invention are to provide novel and improved apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use in a pressure ratio bleed control for a gas turbine engine. As is well known, gas turbine engines may exhibit compressor instability under certain operating conditions. Such instability, also known as compressor surge, occurs when there is a rapid reduction in compressor discharge or burner pressure due to the choking effect of air within the compressor. In order to minimize the effects of compressor surge, it is conventional practice to bleed some of the pressure within the compressor to the ambient atmosphere until such time as the engine passes through the unstable region of its performance characteristic curve.
Prior art pressure ratio bleed controls have provided adaquate performance but have been characterized by a number of disadvantages. Thus, by way of example, the typical prior art control derived a mechanical output from a pneumatic pressure ratio sensor and employed this mechanical output to drive a hydraulic power valve via a gas to liquid converter valve assembly. The necessity of employing a converter valve assembly added to the cost, weight and complexity of the prior art controls while adversely affecting their reliability. Also, prior art pressure ratio bleed controls did not incorporate a simple and convenient override mechanism. A further deficiency of prior art controls was their failure to provide a hysteresis adjustment necessary to compensate for differences in the opening and closing position of the hydraulic valve as commanded by the pneumatic sensor.